Composition of the Earth GLY 4200 Fall, 2016

Interior of the Earth Earth’s interior is divided into zones, with differing properties and compositions Since we live on the crust, it is the most studied The core and mantle are very important in understanding the behavior of the earth Diagram: earthint.gof Division of the earth into crust, mantle, core, and, using a newer system, lithosphere, asthenosphere, mesosphere (transition region on diagram). Transition region is actually 410-660km.

Composition of the Crust – Major Elements Earth’s crust is composed predominantly of eight elements Figure for Si here is correct – figure 5.2 in text has a misprint Numbers are in weight percent Major elements - those with abundance > 1.0wt.% Note: Figure in book says Si = 26.7% - this figure is correct at 27.7%

Abundances Measurements We can specify abundances using differ methods The most common are: Weight per cent Atom per cent Volume percent

Comparison of Methods Element Weight % Atom % O 46.60 62.55 Si 27.72 21.22 Al 8.13 6.47 Fe 5.00 1.92 Ca 3.63 1.94 Na 2.83 2.64 K 2.59 1.42 Mg 2.09 1.82

Minor and Trace Element Definition Minor elements have abundances between 0.1 to 1.0 weight percent Elements with abundances less than 0.1% are called trace elements Their abundance is usually given in parts per million (ppm) or parts per billion (ppb)

Minor and Trace Elements in Crust Only 17 elements occur with abundances of at least 200 parts per million (ppm) – in addition to those on the major element slide, these include: Element Weight % Weight ppm Ti 0.44% F 625 H 0.14% Sr 375 P 0.10% S 260 Mn 0.09% C 200 Ba 0.04%

Ores Many valuable elements are in the trace element range, including the gold group (Au, Ag, and Cu) and the platinum group (Pt, Pd, Ir, Os), mercury, lead, and others Useage does not always reflect abundance – copper (55 ppm) is used more than zirconium (165 ppm) or cerium (60 ppm)

Effect of Pressure As pressure increases, minerals transform to denser structures, with atoms packed more closely together This is seen in the mantle The upper mantle is dominated by the mineral olivine, Mg2SiO4 Magnesium is in VI, and Si in IV

Transition Zone In the transition zone, from about 410 to 660 kilometers below the surface, olivine transforms to denser structures olivine (ρ = 3.22 gm/cm3) → wadsleyite (ρ = 3.47 gm/cm3) → ringwoodite (ρ = 3.55 gm/cm3) Wadsleyite is β- Mg2SiO4 and Ringwoodite is γ-Mg2SiO4, polymorphs of olivine Wadsleyite is a spinelloid, and the structure is based on a distorted cubic-closest packing of oxygen atoms as are the spinels The density values are for Fo analogues – analogues of Fa are heavier

Hydrous Transition Zone The transition zone has been investigated as a hydrous zone within the earth’s mantle Experiments by Ye et al. (2012) on synthetic ringwoodite indicate it can hold about 2.5% water as hydroyxl groups Wadsleyite is also reported to hold up to 3% water This has significant implications for the physical properties and rheology of the transition zone Pearson et al. (2014) found ringwoodite in diamonds from the Juína district of Mato Grosso, Brazil and concluded that this provided direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent water This was the first evidence for the terrestrial occurrence of any higher-pressure polymorph of olivine References: Ye, Y., D.A. Brown, J. R. Smyth, W.R. Panero, S.D. Jacobsen, Y.-Y. Chang, J.P. Townsend, S.M. Thomas, E. Hauri, P. Dera, and D.J. Frost (2012). "Compressibility and thermal expansion study of hydrous Fo100 ringwoodite with 2.5(3) wt% H2O". American Mineralogist 97, 573-582 H on hydroxyl groups replaces Mg2+, or possibly Mg2+ + 2H+ ( on hydroyxl) replace Si4+ D. G. Pearson, F. E. Brenker, F. Nestola, J. McNeill, L. Nasdala, M. T. Hutchison, S. Matveev, K. Mather, G. Silversmit, S. Schmitz, B. Vekemans & L. Vincze (2014), “Hydrous mantle transition zone indicated by ringwoodite included within diamond”, Nature 507, 221-224. Available at http://www.nature.com/nature/journal/v507/n7491/full/nature13080.html “Samples of mantle-derived peridotites show that olivine (Mg2SiO4) is the dominant phase in the Earth’s shallow upper mantle, to a depth of ~400 km. At greater depths, between approximately 410 and 660 km, within the transition zone, the high-pressure olivine polymorphs wadsleyite and ringwoodite are thought to dominate mantle mineralogy owing to the fit of seismic discontinuity data to predictions from phase equilibria. No unretrogressed samples of any high-pressure olivine polymorph have been sampled from the mantle, and, hence, this inference is highly likely, but is unconfirmed by sampling. Sampling the transition zone is important because it is thought to be the main region of water storage in the solid Earth, sandwiched between relatively anhydrous shallow upper mantle and lower mantle. The potential presence of significant water in this part of the Earth has been invoked to explain key aspects of global volcanism and has significant implications for the physical properties and rheology of the transition zone. Finding confirmatory evidence of the presence of ringwoodite in Earth’s mantle, and determining its water content, is an important step in understanding deep Earth processes.

Lower Mantle Pressures are so great that silicon becomes six coordinated (CN = VI), and some magnesium becomes eight-coordinated (perovskite structure) Ringwoodite (ρ = 3.55 gm/cm3) → MgSiO3 (perovskite structure) and (Mg, Fe)O (magnesiowűstite - halite structure) Hydrous ringwoodite article: http://www.nature.com/nature/journal/v507/n7491/full/nature13080.html

Core The core is divided into two regions, the liquid outer core and the solid inner core There is a definite chemical discontinuity between the lower mantle and the outer core The main elements in the core are an iron and nickel alloy Increasing temperature first melts the alloy to make the outer core Increasing pressure freezes the alloy to produce the inner core

Outer Core Ranges from 2900 to 5100 kilometers below the earth Composition is iron with about 2% nickel Density of 9.9 gm/cm3 is too low to be pure metal Best estimates are that silica makes up 9-12% of the outer core

Inner Core From 5100 to 6371 kilometers below surface 80% iron, 20% nickel alloy Pressures reach about 3 megabars, or 300,000 megapascals Temperature at the center is about 7600ºC